The semiconductor industry requires automated wafer handling systems that can precisely handle semiconductor wafers or substrates, such as silicon wafers, of varying diameters, compositions and physical attributes and that can supply those wafers in a rapid, ordered succession to a wafer processing machine. Wafer handling generally entails removing each subject wafer from a wafer cassette, performing preprocessing operations, and individually loading the wafer into the wafer processing machine, followed by returning each processed wafer to a wafer cassette. In high-throughput wafer handling systems, wafers are manipulated or transferred among various locations within the wafer handling system by a robot having a transfer arm equipped with an end effector or a vacuum spatula that is adapted to manipulate and transfer. The wafer handling system typically includes a wafer aligning station for aligning and centering wafers and a load arm for inserting the centered and aligned wafers into a loadlock chamber of the wafer processing machine. The robot, transfer arm and end effector cooperate to place and remove wafers from the load arm, the wafer aligning station, and the wafer cassettes. To that end, the end effector, aligning station, and load arm have chucks, typically vacuum chucks, that securely hold the subject wafer in the vertical, horizontal, and inverted orientations required during the steps of the wafer-handling process.
A conventional chuck contacts an area on the backside or the device side of the wafer that suffices to provide a secure engagement. Certain conventional chucks restrict the contacted area to a narrow annular ring, termed the exclusion area, extending a predetermined distance radially inward from the peripheral rim of the wafer. The predetermined distance typically is less than or equal about 6 millimeters and is unpopulated by devices on the device side of the wafer. The utilization of the exclusion area permits the chucks of wafer handling systems to hold wafers by the device side without damaging devices that may be present during backside treatment and, during device side treatment, to hold wafers by the back side without disturbing any backside treatment. As a result, the exclusion area provides a 6-mm wide annular zone about the peripheral edge that chucks can safely utilize to reliably hold the wafer. One such wafer handling system is described in U.S. Pat. No. 5,820,329, hereby expressly incorporated by reference in its entirety herein.
Silicon wafers are typically manufactured according to standardized specifications which, among other dimensional tolerances, require the surface for receiving devices thereon to be substantially planar with a flatness of 1.5 microns or less. As-manufactured 150 mm silicon wafers, for example, have a standard diameter of 150±0.2 mm and a standard thickness of 675±25 microns. Silicon wafers are usually provided with a flat or a notch used for alignment and indicative of crystalline orientation. Although processed wafers vary in thickness depending upon the end product and the initial wafer diameter, a typical wafer thickness after processing ranges from about 500 microns to about 700 microns. The device fabrication process may introduce additional warpage or bowing that exceeds the flatness in an unprocessed state. For certain end applications, devices and integrated circuits are fabricated on thin silicon wafers having an average thickness of less than or equal to about 150 microns and exhibiting a warpage of as large as about 12.5 mm. Thin wafers are particularly useful in integrated circuit applications, for example, where the heat generated by devices during operation demands an enhanced thermal conduction through the thinned wafer to a heat sink, attached to the back side, in order to prevent overheating of the device side and loss or impairment of functionality.
The vacuum chucks of conventional wafer handling systems, under certain circumstances, are unable to engage the subject wafers in a fashion adequate for secure transfers. Conventional vacuum chucks cannot apply an adequate vacuum distribution over the exclusion area to be chucked. As a result, the wafer is not adequately secured for vertical and inverted orientations of the chuck and may even be dropped by the chuck during movement when the chuck is oriented horizontally. In particular, conventional wafer handling systems and vacuum chucks are unsuited for holding and transferring warped thin semiconductor wafers between locations within the system. In addition, the warpage of thin wafers hinders the accuracy of the alignment and centering operations performed by the aligning station. Therefore, conventional vacuum chucks employed in conventional wafer handling systems cannot apply an adequate gripping force such that thin wafers can be gripped or, if successfully gripped, manipulated about the wafer handling system without a significant risk of dropping the subject wafer.
Accordingly, there is a need for wafer handling systems and wafer handling techniques that can improve the handling of wafers and, in particular, thin wafers, by enhancing the ability of the vacuum chucks of the wafer handling system to secure and hold wafers during handling.